1. Field of the Invention
The present invention relates to an apparatus for diagnosing leakage of evaporative fuel contained in a fuel tank and fed to an intake or induction system of an internal combustion engine.
2. Description of Related Art
For having better understanding of the concept underlying the present invention, technical background thereof will first be reviewed in some detail. FIG. 19 is a schematic diagram showing a structure of a conventional evaporative fuel leak diagnosing apparatus for an internal combustion engine which is disclosed in Japanese Unexamined Patent Application Publication No. 159158/1994 (JP-A-6-159158). Referring to the figure, a fuel tank 1 contains an evaporative fuel 1a. A fuel pipe system 2 is disposed between the fuel tank 1 and an intake or induction manifold (not shown) of the internal combustion engine (not shown either). The fuel pipe system 2 is constituted by fuel pipes 2a, 2b and 2c disposed sequentially in this order as viewed from the side of the fuel tank. A check valve 7 is installed between the fuel pipes 2a and 2b, while installed between the fuel pipes 2b and 2c is a canister 3 for adsorbing fuel vapor. Further, a purge cut valve 4 is provided at the intake manifold side of the engine.
The canister 3 is provided with a drain cut valve 5 for allowing the canister 3 to be selectively communicated to the atmosphere. Additionally, a pressure sensor 6 is mounted on a blanch pipe 2d at a free end thereof. The purge cut valve 4, the drain cut valve 5 and the pressure sensor 6 are electrically connected to a computer unit 50 installed for controlling operations of the internal combustion engine.
Next, description will turn to operations of the evaporative fuel leak diagnosing apparatus.
When the fuel vapor pressure exceeds a preset level at which the check valve 7 is forced to open, the fuel vapor generated within the fuel tank 1 is introduced into the canister 3 by way of the fuel pipe 2a, the check valve 7 and the fuel pipe 2b. The fuel vapor fed to the canister 3 is adsorbed by an adsorbent therein. When the engine is put into operation, the purge cut valve 4 is opened, whereby the fuel adsorbed is purged into the intake manifold of the engine by way of the fuel pipe 2c and the purge cut valve 4. In this manner, fuel vapor generated within the fuel tank 1 ultimately undergoes combustion within the engine cylinders. With the evaporative fuel emission preventing arrangement described above, emission of the evaporative fuel to the atmosphere and hence environmental pollution due to the fuel vapor emission can be prevented.
The pressure sensor 6 is employed for detecting the pressure within the fuel tank 1 in the system-closed-state in which both the drain cut valve 5 and the purge cut valve 4 are closed. The detection signal outputted from the pressure sensor 6 is supplied to the computer unit 50 for detecting the leakage of the fuel vapor by checking whether the rate of pressure increase or rise within the fuel tank 1 exceeds or does not exceed a predetermined value
The pressure sensor 6 used to this end exhibits ordinarily such a linear characteristic as illustrated in FIG. 6.
Needless to say, a fault or defect may occur in the pipes which are usually formed of rubber or the like or at juncture(s) between the pipes and/or parts constituting the evaporative fuel emission preventing apparatus. In that case, the fuel vapor will leak through the faulty or defective portion into the atmosphere. The pressure sensor 6 is provided for the purpose of detecting such leakage and emission of the fuel vapor.
Incidentally, according to the recommendation issued recently by the Environmental Protection Agency of the U.S.A., installation of such fuel vapor detection facility is required for checking whether or not the evaporative fuel emission preventing apparatus is operating normally.
As can readily be understood, the pressure sensor employed in the evaporative fuel leak diagnosing apparatus for the internal combustion engine is required to exhibit a high sensitivity. That is, the pressure sensor should be so designed as to be able to respond to a feeble or minute pressure change with a large output. To this end, a diaphragm constituting a sensing element of the pressure sensor is implemented in a reduced thickness with a large pressure receiving surface (or large diameter) to thereby ensure high sensitivity.
In this conjunction, it is however noted that the reduced thickness of the diaphragm serving as the sensor element involves deterioration in the linearity of the output characteristic of the pressure sensor. For this reason, in the conventional evaporative fuel leak diagnosing apparatus for the internal combustion engine known heretofore, hardwarewise measures are adopted to compensate for deterioration in the linearity of the output characteristic while taking into consideration the S/N ratio of the sensor output signal. Consequently, the conventional evaporative fuel leak diagnosing apparatus is very expensive. Besides, limitation is naturally encountered in the effort for increasing the sensitivity of the pressure sensor.